1. Field of the Invention
This invention relates generally to equipment employed for the coating of substrate articles by magnetron sputtering and, more specifically, to the design of rotatable targets and target mounting spindles used by such equipment, as well as methods and apparatus for affixing such targets to a target mounting spindle.
2. Description of the Prior Art
For at least several decades, sputtering has been the dominant process for the deposition of thin coatings on large-surface-area substrates, such as architectural glass panels, automobile windshields, and the like. The coating materials are deposited with a magnetron in a vacuum chamber containing a reactive gas. Although planar magnetrons were used initially for sputtering processes, rotating cylindrical magnetrons have now come into wide use. This is because cylindrical magnetrons, though more complex than their planar counterparts, typically feature higher deposition rates, higher material utilization per cathode target, lower maintenance frequency, and greater cost efficiency. Another major advantage of using a rotating cylindrical target, as opposed to a planar target, is that dielectric silicon compounds, such as silicon nitride and silicon dioxide, can be sputtered with ease. Such compounds are extremely difficult to sputter using a planar target.
Rotatable cathode targets used in conjunction with magnetron sputtering equipment are generally hollow cylindrical structures which must be supported and rotatably driven. Although very short cylindrical targets may be supported and driven by a single support spindle, as the targets become longer, support at both ends becomes essential. In addition, support at both ends of a cylindrical target simplifies cooling connections, as coolant fluid can enter the target chamber from one support spindle and exit through the other.
There are two basic types of cylindrical cathode targets in use. The first type is an xe2x80x9cunsupportedxe2x80x9d target, a tubular member fabricated from the material to be sputtered. This type of target is limited to those materials having the mechanical strength for self support over the target length. Another factor which restricts the use of certain materials in unsupported targets is whether or not the material can be readily formed into the target shape. Materials, such as structural metals, that are easily machined and formed, are typical candidates for unsupported targets. Titanium, stainless steel and niobium fall into this category. The second type of cylindrical cathode target, known as a supported target, is used for the sputtering of materials which have poor structural characteristics such as low mechanical strength and softness or characteristics which hamper machining or forming, such as brittleness or extreme hardness. Materials which are generally sputtered from a supported target include gold, silver, copper, manganese, silicon and certain ceramics. These materials must be deposited on a cylindrical backing tube fabricated from a material having sufficient mechanical strength to support both itself and the sputterable material it carries over the target length. Typically, cylindrical backing tubes, or carriers, are manufactured from stainless steel. Deposition of the supported material may be accomplished via one of various known techniques, such as plating, sputtering, or dipping the carrier tube in molten sputterable material. In order to avoid confusion of target genre with the need to support a rotatable target at one or both ends, unsupported targets will hereinafter be referred to as xe2x80x9cself-carryingxe2x80x9d targets. For the same reason, supported targets, which require the use of a backing tube, will hereinafter be referred to as xe2x80x9ccarriedxe2x80x9d targets.
Various mechanisms for supporting, rotating, cooling and energizing a cylindrical target structure in a magnetron, are described in U.S. Pat. No. 5,096,562 to Boozenny, et al. The structure employs a removable support spindle at each end of the cylindrical target structure. A first support spindle at one end supplies and withdraws cooling fluid from within the cylinder and contains a driving mechanism to rotate it, while a second support spindle at the other end contains the electrical power connection to the target surface.
An improved apparatus to releasably affix a rotatable cylindrical magnetron target to a support spindle is disclosed in U.S. Pat. No. 5,591,314 to Morgan, et al. The inventors claim that a primary disadvantage of the Boozenny, et al. design is frequent breakdowns caused by coolant leaks at the interface between the first support spindle and the cylindrical target structure. The leaks are due to the fact that one side of any seal at the spindle-to-target interface is subjected to the low pressure present within the deposition chamber, while the other side of the seal is subjected to high coolant pressure. The Morgan, et al. apparatus supposedly improves on the Boozenny, et al. apparatus by providing a removable coiled wire thread connection system, by employing a single coolant seal ring, to which uniform pressure is applied as the target is attached to the support spindle. For a preferred embodiment, the target, the spindle, and the coiled wire thread are made from dissimilar materials.
Though the Morgan, et al. apparatus seems to be a significant improvement over the Boozenny, et al. apparatus, there are several disadvantages associated with the design of the former. Firstly, the use of the coiled wire thread requires that a spiral groove be machined in the cylindrical target body, thereby weakening the structure. The thickness of the target cylinder at the deepest point of the groove is only about 1.0 millimeter (0.030 inch). Secondly, the target body sealing surface is on a circumferential edge thereof. As the edge has little surface area, it is easily nicked. Nicks in the sealing surface may well cause coolant leaks. Thirdly, the coupling arrangement between target and spindle does not permit the transfer of large torque loads. Fourthly, the threaded coupling arrangement does not lend itself to evenly distributed clamping forces between the target and the spindle.
What is needed is are new cylindrical target and spindle designs which eliminate the disadvantages of the prior art targets and spindles.
The present invention includes a method and apparatus for attaching a rotating cylindrical magnetron cathode target to an existing, rotatable flanged support spindle. Because the invention may be applied to cylindrical targets of both the carried and self-carrying types, the word xe2x80x9ctargetxe2x80x9d should be understood to include a xe2x80x9ctarget carrierxe2x80x9d unless the latter is specifically excluded. The invention may also be practices with targets that are supported at one or both ends. It should also be understood that if a target is supported at both ends, the attachment method and apparatus for both ends are either identical or mirror images of one another. Several design improvements have been incorporated into the magnetron target of the present invention, and the attachment apparatus is specifically designed to couple the improved magnetron target to an existing flanged support spindle.
The objectives in creating the improved magnetron target and attachment apparatus have been to improve the reliability of a rotating cylindrical target or target carrier by reducing the likelihood of coolant leaks between the support spindle and the target, decrease the time required to change out an installed target, improve the structural integrity of the target, and provide a target-spindle unifying clamp system which provides more even distribution of clamping pressure, thereby permitting the transfer of much larger torque loads from the spindle to the target.
The improved target includes a circumferential shoulder which, for a preferred embodiment of the invention, includes an extensive circumferential lip which fits over the spindle""s outer mating circumferential edge. This lip affords additional protection to a circumferential sealing surface on the target, so that it is less likely to become mechanically damaged. The shoulder terminates in a circumferential, inwardly-angled, overhanging step. The target and spindle are coupled together with an generally annular retainer ring having external, outwardly-facing circumferential threads, and a split clamping collar having an inwardly-angled clamping ledge at one end thereof that mates with the overhanging circumferential step on the target, and internal, inwardly-facing circumferential threads at the other end, which mate with the external threads of the retainer ring. The retainer ring is rotated with a spanner wrench so as to engage the threads of the split clamping collar. As the retainer ring engages the rear face of the spindle flange, the clamping collar and retainer ring act in concert to draw the target and spindle together as a unified assembly. As the target and spindle are drawn together, the circumferential sealing edge of the target compresses an O-ring seal set within an annular groove in the spindle.